Midterm Prep.docx

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School

Ryerson University

Department

Information Technology Management

Course

ITM 301

Professor

Franklyn Prescod

Semester

Fall

Description

Chapter 1
Server (Host Computer) – stores data or software that can be accessed by the clients
Client – the input-output hardware device at the user’s end of a communication circuit
Circuit – the pathway through which the messages travel
Telecommunications - Transmission of voice, video, and/or data
Data Communications - Movement of computer information by means of electrical or optical
transmission systems
• Local Area Networks (LAN) - room, building
– a group of PCs that share a circuit.
• Backbone Networks (BN) - less than few kms
– a high speed backbone linking together organizational LANs at various locations.
• Metropolitan Area Networks (MAN) - (more than a few kms)
– connects LANs and BNs across different locations
– Often uses leased lines or other services used to transmit data.
• Wide Area Networks (WANs) - (far greater than 10 kms)
– Same as MAN except wider scale
• Intranet
– A LAN that uses the Internet technologies within an organization
– Open only those inside the organization
– Example: insurance related information provided to employees over an intranet
• Extranet
– A LAN that uses the Internet technologies across an organization including some
external constituents
– Open only those invited users outside the organization
– Accessible through the Internet
– Example: Suppliers and customers accessing inventory information in a company over
an extranet
Page 1 of 31 2 Important Multi-layer network models
• Open Systems Interconnection Model
– Created by International Standards Organization (ISO) as a framework for computer
network standards in 1984
Based on 7 layers
1. Application Layer - set of utilities used by application programs
2. Presentation Layer - formats data for presentation to the user
 provides data interfaces, data compression and translation between different data
formats
3. Session Layer - initiates, maintains and terminates each logical session between sender and
receiver
4. Transport Layer - deals with end-to-end issues such as segmenting the message for network
transport, and maintaining the logical connections between sender and receiver
5. Network Layer - responsible for making routing decisions
6. Data Link Layer - deals with message delineation, error control and network medium access
control
7. Physical Layer - defines how individual bits are formatted to be transmitted through the
network
• Internet Model
– Created by DARPA originally in early 70’s
– Developed to solve to the problem of internetworking
– Layers allow simplicity of networking in some ways
• Easy to develop new software that fits each layer
• Relatively simple to change the software at any level
• Matching layers communicate between different computers and computer platforms
– Accomplished by standards that we all agree on
– e.g., Physical layer at the sending computer must match up with the same layer in the
receiving computer
• Somewhat inefficient
– Involves many software packages and packets
– Packet overhead (slower transmission, processing time)
– Interoperability (ability to exchange and use information) achieved at the expense of
perfectly streamlined communication
Based on 5 layers - Based on Transmission Control Protocol/ Internet Protocol (TCP/IP) suite
1. Application Layer - used by application program
2. Transport Layer - responsible for establishing end-to-end connections, translates domain names
into numeric addresses and segments messages
3. Network Layer - responsible for making routing decisions (Chooses computer) and find address
of that computer
4. Data Link Layer – responsible for moving a message from one computer to the next computer in
the network path from the sender to the receiver. Performs 3 functions:
 Controls the physical layer by deciding when to transmit messages over the media
 Formats messages by indicating where they start and end
 Detects and corrects errors that have occurred during transmission
5. Physical Layer – physical connection between the sender and receiver (Hardware, circuits)
Page 2 of 31 Protocols - Sets of standardized rules to define how to communicate at each layer and how to interface
with adjacent layers
• Used by Network model layers
Standards (Importance) - Provide a “fixed” way for hardware and/or software systems (different
companies) to communicate
– Help promote competition and decrease the price
Types of Standards
 Formal standards
o Developed by an industry or government standards-making body
 De-facto standards
o Emerge in the marketplace and widely used
o Lack official backing by a standards-making body
 Standardization Process
o Specification - Developing the nomenclature and identifying the problems to be
addressed
o Identification of choices - Identifying solutions to the problems and choose the
“optimum” solution
o Acceptance - Defining the solution, getting it recognized by industry so that a uniform
solution is accepted
 Major Standard Bodies
o ISO (International Organization for Standardization)
 Technical recommendations for data communication interfaces
 Composed of each country’s national standards orgs.
 Based in Geneva, Switzerland (www.iso.ch)
o ITU-T (International Telecommunications Union –Telecom Group
 Technical recommendations about telephone, telegraph and data
communications interfaces
 Composed of representatives from each country in UN
 Based in Geneva, Switzerland (www.itu.int)
o ANSI (American National Standards Institute)
 Coordinating organization for US (not a standards- making body)
Page 3 of 31  www.ansi.org
o IEEE (Institute of Electrical and Electronic Engineers)
 Professional society; also develops mostly LAN standards
 standards.ieee.org
o IETF (Internet Engineering Task Force)
 Develops Internet standards
 No official membership (anyone welcome)
 www.ietf.org
Emerging Trends:
 Pervasive Networking - Networks will be everywhere
 Convergence - Networks that were previously transmitted using separate networks will merge
into a single, high speed, multimedia network in the near future
Application Service Providers (ASPs) -Develop specific systems for companies to use; such as providing
and operating a payroll system for a company that does not have one of its own
 Information Utilities (Future of ASPs) - Providing a wide range of info services (email, web,
payroll, etc.) (similar to electric or water utilities)
Page 4 of 31 Chapter 2
Application Layer - Introduction
• Application architecture
• The way in which the functions of the application layer software are spread among the
clients and servers on the network
• Functions of Application Layer
• Data storage - Storing of data generated by programs (e.g., files, records)
• Data access logic - Processing required to access stored data (e.g., SQL)
• Application logic - Business logic such as word processors, spreadsheets
• Presentation logic - Presentation of info to user & acceptance of user commands
Four Application Architectures:
Host-based Architectures - Server performs almost all functions
 Problems: Host becoming a bottleneck - All processing done by the host, which can severely
limit network performance
 Host upgrades typically expensive and “lumpy”
o Available upgrades require large scale and often costly jumps in processing and memory
o Network demand grows more incrementally than does the host capacity
o May see poor fit (too much or too little) between host performance and network
demand
Client-based architectures - Client performs most functions
Page 5 of 31  Data traffic must travel back and forth between server and client
– Example: when the client program is making a database query, the ENTIRE database
must travel to the client before the query can be processed
– Often the large file sizes moving across the LAN can yield a poor result in network
performance
Client-server architectures - Functions shared between client and server
• Advantages
– More efficient because of distributed processing
– Allow hardware and software from different vendors to be used together
• Disadvantages
– Difficulty in getting software from different vendors to work together smoothly
– May require Middleware, a third category of software
Peer-to-Peer architectures – computers are both clients and servers and thus share the work
• All computers can serve as a client and a server
• Increased popularity in the last decade due to the rise of P2P services such as Napster
• Advantages:
• Data can be stored anywhere on the network
• Very resilient to failure
• Disadvantages:
• Finding data
• Security
Middleware - a standard way of translating between software from different vendors
– Manages message transfers
– Insulates network changes from the clients (e.g., adding a new server)
Examples of standards for Middleware:
– Distributed Computing Environment (DCE)
– Common Object Request Broker Architecture (CORBA)
– Open Database Connectivity (ODBC)
Multi-tier Architectures
• Involve more than two computers in distributing application program logic
Page 6 of 31 – 2-tier architecture
• Uses clients and servers in a balance, very popular approach in simple LANs
– 3-tier architecture
• 3 sets of computers involved
– N-tier architecture
• More than three sets of computers used, more typical across complex
organizations
• Allows load balancing across servers
• Advantages
– Better load balancing:
• More evenly distributed processing. (e.g., application logic distributed between
several servers.)
– More scalable:
• Only servers experiencing high demand need be upgraded
• Disadvantages
– Heavily loaded network:
• More distributed processing necessitates more data exchanges
– Difficult to program and test due to increased complexity
Thin and Thick Clients -Classification depends on how much of the application logic resides on the client
or server
• Thin client - Little or no application logic on client
• Becoming popular because easier to manage, (only the server application logic
generally needs to be updated)
• The best example: World Wide Web architecture (uses a two-tier, thin client
architecture)
• Thick client - All or most of the application logic resides on the client
Criteria for Choosing Architecture
• Infrastructure Cost
– Cost of servers, clients, and circuits
– Mainframes: very expensive; terminals, PCs: inexpensive
• Development Cost
– Mainly cost of software development
– Software: expensive to develop; off-the-shelf software: inexpensive
• Scalability
– Ability to increase (or decrease) in computing capacity as network demand changes
– Mainframes: not scalable; PCs: highly scalable
Page 7 of 31 Uniform Resource Locators (URLs) - A formal way of identifying links to other documents
HTML (Hypertext Markup Language) - A language used to create Web pages
• Also developed at CERN (initially for text files)
• Tags are embedded in HTML documents
– include information on how to format the file
• Extensions to HTML needed to format multimedia files
• XML - Extensible Markup Language - A new markup language becoming popular
E-Mail standards:
SMTP (Simple Mail Transfer Protocol) - Main e-mail standard for
 Originating user agent and the mail transfer agent
 Between mail transfer agents
 Originally written to handle only text files
 Usually used in two-tier client-server architectures
Post Office Protocol (POP) and Internet Mail Access Protocol (IMAP)
 Main protocols used between the receiver user agent and mail transfer agent
 Main difference: with IMAP, messages can be left at the server after downloading them to the
client
Other competing standards
 Common Messaging Calls (CMC), X.400
Two-Tier E-mail Architecture
 User agent is another word for an e-mail client application
o Run on client computers
o Send e-mail to e-mail servers
o Download e-mail from mailboxes on those servers
Page 8 of 31 o Examples: Eudora, Outlook, Netscape Messenger
 Mail transfer agent is another word for the mail server application
o Used by e-mail servers
o Send e-mail between e-mail servers
o Maintain individual mailboxes.
Host Based e-mail Architectures
• An old method used on UNIX based hosts
• Similar to client-server architecture, except
– Client PC replaced by a terminal (or terminal emulator)
• Sends all keystrokes to the server
• Display characters received from the server
– All software resides on the server
• Takes client keystrokes and understand user’s commands
• Creates SMTP packets and sends them to next mail server
• Useful when traveling in locations with poor internet facilities
Multipurpose Internet Mail Extension (MIME) - A graphics capable mail transfer agent protocol (to send
graphical information in addition to text)
o SMTP was designed years ago for text transfer only
– MIME software is included as part of an e-mail client
– Translates graphical information into text allowing the graphic to be sent as part of an
SMTP message (as a special attachment)
– Receiver’s e-mail client then translates the MIME attachment from text back into
graphical format
– MIME example
Listserv Discussion Groups - Mailing lists of users who join to discuss some special topic (e.g., cooking,
typing, networking)
File Transfer Protocol (FTP) - Enables sending and receiving files over the Internet
• Requires an application program on the client computer and a FTP server program on a server
• Commonly used today for uploading web pages
• Many packages available using FTP
– WS-FTP (a graphical FTP software)
• FTP sites: Closed sites - Requires account name and password | Anonymous sites - Account
name: anonymous; password: email address
Telnet - Allows one computer to log into another computer
– Remote login enabling full control of the host
• Requires an application program on the client computer and a Telnet server program on the
server
– Client program emulates a “dumb” terminal off the server
• Many packages available conforming Telnet
– EWAN
• Requires account name and password
– Anonymous sites similar to FTP approach
• Account name: anonymous; password: email address
• Instant Messaging (IM) - A client-server program that allows real-time typed messages to be
exchanged
– Client needs an IM client software
– Server needs an IM server package
Page 9 of 31 • Some types allow voice and video packets to be sent
– Like a telephone
• Examples include AOL and ICQ
• Two step process:
– Telling IM server that you are online
– Chatting
Videoconferencing - Provides real time transmission of video and audio signals between two or more
locations
– Allows people to meet at the same time in different locations
– Saves money and time by not having to move people around
– Typically involves matched special purpose rooms with cameras and displays
• Desktop videoconferencing
– Low cost application linking small video cameras and microphones together over the
Internet
– No need for special rooms
– Example: Net Meeting software on clients communicating through a common
videoconference server
• Common standards in use today
– H.320 - Designed for room-to-room videoconferencing over high-speed phone lines
– H.323 - Family of standards designed for desktop videoconferencing and just simple
audio conferencing over Internet
– MPEG-2 - Designed for faster connections such as LAN or privately owned WANs
• Webcasting - Special type of uni-directional videoconferencing
– Content is sent from the server to users
• Process
– Content created by developer
– Downloaded as needed by the user
– Played by a plug-in to a Web browser
• No standards for webcasting yet
– Defacto standards: products by RealNetworks
Web Has 2 application software packages: a web browser on the client, and a web server on the server
 They communicate using a HTTP
 Most web pages written in HTML but also use other formats
Page 10 of 31 Chapter 3
Physical Layer Overview
• Includes network hardware and circuits
• Network circuits:
– physical media (e.g., cables) and
– Special purposes devices (e.g., routers and hubs).
• Types of Circuits
– Physical circuits connect devices & include actual wires such as twisted pair wires
– Logical circuits refer to the transmission characteristics of the circuit, such as a T-1
connection refers to 1.544 Mbps
– Physical and logical circuits may be the same or different. For example, in multiplexing,
one physical wire may carry several logical circuits.
Analog data
– Produced by telephones
– Sound waves, which vary continuously over time, analogous to one’s voice
– Can take on any value in a wide range of possibilities
Digital data
– Produced by computers, in binary form
– information is represented as code in a series of ones and zeros
– All digital data is either on or off, 0 or 1
Types of Transmission
• Analog transmissions
– Analog data transmitted in analog form
– Examples of analog data being sent using analog transmissions are broadcast TV and
radio
• Digital transmissions (Baseband transmission)
– Made of discrete square waves with a clear beginning and ending
– Computer networks send digital data using digital transmissions
• Data converted between analog and digital formats
– Modem (modulator/demodulator): used when digital data is sent as an analog
transmission
– Codec (coder/decoder): used when analog data is sent via digital transmission
Digital Transmission: Advantages
• Produces fewer errors - Easier to detect and correct errors, since transmitted data is binary (1s
and 0s, only two distinct values)
– A weak square wave can easily be propagated again in perfect form, allowing more crisp
transmission than analog
• Permits higher maximum transmission rates - e.g., Optical fiber designed for digital transmission
• More efficient - Possible to send more digital data through a given circuit, circuit can be
“packed”
• More secure - Easier to encrypt digital bit stream
• Simpler to integrate voice, video and data - Easier mix and match V, V, D on the same circuit,
since all signals made up of 0’s and 1’s
Circuit Configuration
• Basic physical layout of the circuit
Page 11 of 31 • Configuration types:
– Point-to-Point Configuration
• Goes from one point to another
• Sometimes called “dedicated circuits”
• Used when computers generate enough data to fill the capacity of the circuit
• Each computer has its own circuit to reach the other computer in the network
(expensive)
– Multipoint Configuration
• Many computer connected on the same circuit
• Sometimes called “shared circuit”
• Used when each computer does not need to continuously use the entire
capacity of the circuit
• Cheaper (not as many wires) and simpler to wire
• Only one computer can use the circuit at a time
Selection of Data Flow Method
– Simplex Method -If data required to flow in one direction only
• e.g., From a remote sensor to a host computer
– Half-Duplex Method – Take turns so that the data required flow in one direcectopn and
then in the other
• Terminal-to-host communication (send and wait type communications)
– Full Duplex Method – Data flows in both direction
• Client-server; host-to-host communication (peer-to-peer communications)
• Capacity may be a factor too
– Full-duplex uses half of the capacity for each direction
Multiplexing - Breaking up a higher speed circuit into several slower (logical) circuits
– Several devices can use it at the same time
– Requires two multiplexer: one to combine; one to separate
• Main advantage: cost
– Fewer network circuits needed
• Categories of multiplexing:
– Frequency division multiplexing (FDM) - Makes a number of smaller channels from a
larger frequency band by dividing the circuit “horizontally”
– Time division multiplexing (TDM) - Dividing the circuit “vertically”
Page 12 of 31 • TDM allows terminals to send data by taking turns
• This example shows 4 terminals sharing a circuit, with each terminal sending
one character at a time
• Time on the circuit shared equally
• Each terminal getting a specified timeslot whether needed or not
• More efficient than FDM
• Since TDM doesn’t use guardbands, entire capacity can be divided up between
terminals
– Statistical time division multiplexing (STDM) - Designed to make use of the idle time
slots
• In TDM, when terminals are not using the multiplexed circuit, timeslots for
those terminals are idle
• Uses non-dedicated time slots
• Time slots used as needed by the different terminals
• Complexities of STDM
• Additional addressing information needed
• Since source of a data sample is not identified by the time slot it occupies
• Potential response time delays (when all terminals try to use the multiplexed
circuit intensively)
• Requires memory to store data (in case more data comes in than the outgoing
circuit capacity can handle)
- Wavelength division multiplexing (WDM) - Transmitting data at many different
frequencies
o Lasers or LEDs used to transmit on optical fibers
o Previously single frequency on single fiber (typical transmission rate being
around 622 Mbps)
o Now multi frequencies on single fiber  n x 622+ Mbps
o Dense WDM (DWDM)
o Over a hundred channels per fiber
o Each transmitting at a rate of 10 Gbps
o Aggregate data rates in the low terabit range (Tbps)
o Future versions of DWDM
o Both per channel data rates and total number of channels continue to rise
o Possibility of petabit (Pbps) aggregate rates
Inverse Multiplexing (IMUX) - Shares the load by sending data over two or more lines
• Bandwidth ON Demand Network Interoperability Group (BONDING) standard
• Commonly used for videoconferencing applications
• Six 64 kbps lines can be combined to create an aggregate line of 384 kbps for transmitting video
Digital Subscriber Line (DSL) - Became popular as a way to increase data rates in the local loop.
– Uses full physical capacity of twisted pair (copper) phone lines (up to 1 MHz) instead of
using the 0-4000 KHz voice channel
xDSL
• Several versions of DSL
– Depends on how the bandwidth allocated between the upstream and downstream
channels
– A for Asynchronous, H for High speed, etc
• G.Lite - a form of ADSL
Page 13 of 31 – Provides
• a 4 Khz voice channel
• 384 kbps upstream
• 1.5 Mbps downstream (provided line conditions are optimal).
Communications Media: Physical matter that carries transmission
- Guided media - Transmission flows along a physical guide (media guides the signal across
the network) Examples include twisted pair wiring, coaxial cable and fiber optic cable
- Wireless media (radiated media) - No wave guide, the transmission flows through the
air or space Examples include radio such as microwave and satellite, as well as infrared
communications
Twisted Pair (TP) Wires - Commonly used for telephones and LANs
• Reduced electromagnetic interference
– Via twisting two wires together
(Usually several twists per inch)
• TP cables have a number of pairs of wires
– Telephone lines: two pairs (4 wires, usually only one pair is used by the telephone)
– LAN cables: 4 pairs (8 wires)
• Also used in telephone trunk lines (up to several thousand pairs)
• Shielded twisted pair also exists, but is more expensive
Coaxial Cable
• Less prone to interference than TP due to shield
• More expensive than TP, thus quickly disappearing
• Used mostly for cable TV
Fiber Optic Cable
• Light created by an LED (light-emitting diode) or laser is sent down a thin glass or plastic fiber
• Has extremely high capacity, ideal for broadband
• Works well under harsh environments
– Not fragile, nor brittle; Not heavy nor bulky
– More resistant to corrosion, fire, water
– Highly secure, know when is tapped
• Fiber optic cable structure (from center):
– Core (v. small, 5-50 microns, ~ the size of a single hair)
– Cladding, which reflects the signal
Protective outer jacket
Types of Optical Fiber
• Multimode (about 50 micron core)
– Earliest fiber-optic systems
– Signal spreads out over short distances (up to ~500m)
– Inexpensive
• Graded index multimode
– Reduces the spreading problem by changing the refractive properties of the fiber to
refocus the signal
– Can be used over distances of up to about 1000 meters
• Single mode (about 5 micron core)
Page 14 of 31 – Transmits a single direct beam through the cable
– Signal can be sent over many miles without spreading
– Expensive (requires lasers; difficult to manufacture)
Wireless Media
 Radio
o Wireless transmission of electrical waves through air
o Each device has a radio transceiver with a specific frequency
• Low power transmitters (few miles range)
• Often attached to portables (Laptops, PDAs, cell phones)
– Includes
• AM and FM radios, Cellular phones
• Wireless LANs (IEEE 802.11) and Bluetooth
• Microwaves and Satellites, Low Earth Orbiting Satellites
2.) Infrared
– “invisible” light waves with frequency below red light spectrum
– Requires line of sight; generally subject to interference from heavy rain, smog, and fog
– Used in remote control units such as for controlling the TV
3.) Microwave Radio
• High frequency form of radio communications
– Extremely short (micro) wavelength (1 cm to 1 m)
– Requires line-of-sight
• Performs same functions as cables
– Often used for long distance, terrestrial transmissions (over 50 miles without
repeaters)
– No wiring and digging required
– Requires large antennas (about 10 ft) and high towers
• Possesses similar properties as light
– Reflection, refraction, and focusing
– Can be focused into narrow powerful beams for long distance
– Some effect from water, rain and snow
4.) Satellite Communications
– Special form of microwave communications
– Signals travel at speed of light, yet long propagation delay
– due to great distance between ground station and satellite
Factors Used in Media Selection
• Type of network - LAN, WAN, or Backbone
• Cost - Always changing; depends on the distance
• Transmission distance - Short: up to 300 m; medium: up to 500 m
• Security - Wireless media is less secure
• Error rates - Wireless media has the highest error rate (interference)
• Transmission speeds - Constantly improving; Fiber has the highest
Page 15 of 31 Digital Transmission of Digital Data
• Computers produce binary data
• Standards needed to ensure both sender and receiver understands this data
– Codes: digital combinations of bits making up languages that computers use to
represent letters, numbers, and symbols in a message
– Signals: electrical or optical patterns that computers use to represent the coded bits (0
or 1) during transmission across media
• ASCII: American Standard Code for Information Interchange
– Originally used a 7-bit code (128 combinations), but an 8-bit version (256 combinations)
is now in use
– Found on PC computers
• EBCDIC: Extended Binary Coded Decimal Interchange Code
– An 8-bit code developed by IBM
– Used mostly in mainframe computer environment
Transmission Modes
• Bits in a message can be sent on:
– a single wire one after another (Serial transmission)
– multiple wires simultaneously (Parallel transmission)
• Serial Mode
– Sends bit by bit over a single wire
– Serial mode is slower than parallel mode
– Can be used over longer distances since bits stay in the order they were sent
• Parallel mode
– Uses several wires, each wire sending one bit at the same time as the others
• A parallel printer cable sends 8 bits together
• Computer’s processor and motherboard also use parallel busses (8 bits, 16 bits,
32 bits) to move data around
• Used for short distances (up to 6 meters) since bits sent in parallel mode tend
to spread out over long distances
Signaling of Bits
• Digital Transmission
– Signals sent as a series of “square waves” of either positive or negative voltage
– Voltages vary between +3/-3 and +24/-24 depending on the circuit
• Signaling (encoding)
– Defines how the voltage levels will correspond to the bit values of 0 or 1
Page 16 of 31 – Examples:
• Unipolar, Bipolar
• RTZ, NRZ, Manchester
– Data rate: describes how often the sender can transmit data
• 64 Kbps  once every 1/64000 of a second
Signaling (Encoding) Techniques
• Unipolar signaling (Digital Transmission)
– Use voltages either vary between 0 and a positive value or between 0 and some
negative value
• Bipolar signaling
– Use both positive and negative voltages
– Experiences fewer errors than unipolar signaling
• Signals are more distinct (more difficult for interference to change polarity of a
current)
– Return to zero (RZ)
• Signal returns to 0 voltage level after sending a bit
– Non return to zero (NRZ)
• Signals maintains its voltage at the end of a bit
– Manchester encoding (used by Ethernet)
• Used by Ethernet, most popular LAN technology
• Defines a bit value by a mid-bit transition
• A high to low voltage transition is a 0 and a low to high mid-bit transition
defines a 1
• Data rates: 10 Mb/s, 100 Mb/s, 1 Gb/s
• 10- Mb/s  one signal for every 1/10,000,000 of a second (10 million signals
or bits every second)
• Less susceptible to having errors go undetected
• If there is no mid-bit voltage transition, then an error took place
Analog Transmission of Digital Data
• A well known example using phone lines to connect PCs to the Internet
• PCs generate digital data
• Local loop phone lines use analog transmission technology
• Modems translate digital data into analog signals
Telephone Network
• Originally designed for human speech (analog communications) only
• POTS (Plain Old Telephone Service)
– Enables voice communications between two telephones
–